From John Nack's blog: "In the interest of giving customers guidance as early as possible, we have some news to share on this point: in addition to offering 32-bit-native versions for Mac OS X and 32-bit Windows, just as we do today, we plan to ship the next version of Photoshop as 64-bit-native for Windows 64-bit OSes only."

In the general case, yes. When talking about x86 processors, not true. In 32 bit mode, an x86 processor has 8 general purpose registers to work with. That's not much, and limits the efficiency of code. When in 64 bit mode, you've got 16 registers. You get about a 20% speed increase just from that. If we were still talking PowerPC here, then yeah, 64 bit wouldn't help.

64bit instructions mostly only benefit if you have to do lots of maths. In the case of CS it would likely only benefit things like filters, and even then only marginally.

What do you think filters are, if not lots of math?

Yeah, 64 bit helps when doing math on huge numbers - which Photoshop probably doesn't do - but the bigger help is when using lots of memory, which Photoshop is known for.